The shipment of heat-sensitive bio-systems, as for instance semen, vaccines, cultures of bacteria and viruses at optimal temperature levels between about 78 K. and 100 K., poses a series of difficulties. The vials or "straws", in which the biologicals are hermetically sealed, must be kept continuously at near liquid nitrogen temperature to preserve the viability of the biological product. But since the boiling point of liquid nitrogen at ambient pressure is 77.4 K. (-320.4.degree. F.) the cryogen holding vessel (refrigerator) must remain open to the atmosphere to vent the boiled-off gas and thus avoid a dangerous pressure build-up inside. For this reason open-to-atmosphere liquid nitrogen vessels are used for refrigeration. It is obvious that such vessels must be kept upright at all times to prevent spillage of the cryogen. This condition is difficult to control during a long shipment unless an attendant accompanies the vessel on the trip which is rarely a feasible option.
To overcome the difficulties associated with the shipment of biologicals at cryogenic temperature a shipping container was developed in which the liquid nitrogen is retained in a solid porous mass by adsorption, capillarity and absorption. Based upon this development a patent issued to R. F. O'Connell et al. in 1966 as U.S. Pat. No. 3,238,002. The shipping container described in this patent is of a double-walled construction to provide a vacuum space around the inner vessel which holds the liquid nitrogen. The vacuum space is filled with a multilayer insulation to reduce heat transfer by radiation. An adsorbent and a getter are part of the system to maintain vacuum integrity. The inner vessel is filled with the solid porous mass which, when saturated with liquid nitrogen, will hold the cryogen by adsorption, and capillarity as well as by absorption, similar to a sponge "holding" water. In the center of the porous filler core one or more voids are provided to hold the vials containing the biologicals.
The solid components of the porous mass described in U.S. Pat. No. 3,238,003 are silica (sand), quick-lime, and a small amount of inert heat resistant mineral fibers such as asbestos. The porous mass is formed starting with an aqueous slurry of the filler components which is poured into a mold and then baked in an autoclave under precisely controlled equilibrium conditions of pressure and temperature. The components undergo a chemical reaction forming a porous mass of calcium silicates, reinforced by inert fibers. The evaported water leaves inside the dried out solid structure microscopic voids, of complex geometry, sometimes referred to as "pores", which comprise on the average 89.5% of the apparent solid volume. Since the resulting mass is incompressible the mold must either provide the mass with a shape conforming to the inner vessel of the storage container or it must be machined to size. The porous mass is filled with liquid nitrogen by submerging it in a liquid nitrogen bath until it is saturated. The filling operation for a conventional two liter container housing a sand-lime porous mass matrix takes about twenty-four hours.
The baked sand-lime porous mass is intrinsically hydrophilic. Because of this property moisture must be periodically driven out of the porous mass matrix to prevent the accumulation of trapped water. If this is not done, the trapped water will turn into ice crystals every time it is exposed to liquid nitrogen and eventually will crack the brittle microstructure of the filler. This may be prevented by periodically heating the porous structure to above 100.degree. C. after several fill and warm up cycles.
Although the ingredients used in manufacturing the sand-lime porous mass are relatively inexpensive (deionized water, sand, quick-lime and inert fibers, as for example asbestos) the finishing operations in handling a solid porous mass are very expensive due to the high labor costs involved and the elaborate safety precaustions required. It is not economically feasible to cast the porous filler in a cryogenic holding vessel. Elaborate safety precautions are indispensable when handling substances like asbestos fibers and noxious dust. In addition, the thermal energy cost is very high for the manufacturing process of the sand-lime filler mass.
Alternative systems for retaining liquid nitrogen in a storage container through a combination of adsorption, absorption and capillarity have in the past being investigated by those skilled in the art. The use of high porosity blocks, artificial stones, bricks and light papers made from cellulose fibers such as towels and bathroom tissues have been studied and, in general have been dismissed as inferior compared to the use of the sand-lime porous mass matrix due primarily to their low porosity. The average porosity of the sand-lime porous matrix is 89.5% whereas the porosity of a matrix fabricated from any of the aforementioned materials is below 60%. More recently block insulation material composed of hydrous calcium silicate has been used as the adsorption matrix. Such material is closer in porosity to the sand-lime porous mass composition but also has most of the shortcomings of the sand-lime porous mass composition. The porosity of the filler matrix determines for a given size shipping container its liquid nitrogen capacity. The porosity and rate of evaporation are the most important characteristics of a liquid nitrogen storage container for transporting a product at cryogenic temperatures. A storage container using a sand-lime porous mass matrix has an average 5 day holding time based on an evaporation rate of 0.33 liters per day and a liquid capacity of 1.6 liters.
Accordingly, the art has long sought a less expensive and much more efficient liquid nitrogen adsorption system as an alternative to the storage systems in present use.